In fluid processing systems in which both gas and liquid phases flow through a pipeline or conduit, slug flow or slugging can occur when a large volume of gas or of liquid known as a slug travels through the pipeline. When liquid or gas slugs exit the pipeline at a processing facility, they can be produced at a rate which exceeds the ability of the fluid handling equipment to accommodate. Slugging behavior can be categorized as hydrodynamic slugging, terrain slugging, riser slugging, or operational slugging. Combinations of these types of slugging behavior can also occur. Hydrodynamic slugging is a known multiphase flow regime that occurs at certain values of superficial gas and liquid velocities. Terrain slugging can be caused by the changes in elevation in a pipeline, e.g., in the case of subsea pipeline, a pipeline along an uneven seabed with large variations in elevation. The liquid phase accumulates at a low point until the pipe is filled with liquid at that point. Then, gas accumulates behind the liquid slug until sufficient pressure builds up to move the slug of liquid through the pipeline. Riser-based slugging is a special form of terrain slugging associated with pipeline risers used in offshore oil production. Liquid accumulates at a low point or a bend in the riser to form a liquid slug which is produced once sufficient gas pressure is built up behind it to push the liquid slug over the top of the riser. Operational slugging can be caused by operational changes, such as start-up of a wet gas facility, and is most often handled via ramp-up of the facility.
Various schemes have been tried to control slugging behavior in fluid processing systems, including subsea fluid processing systems. The accepted control approaches to the various categories of slugging are different, because the causes of the behaviors are different, although all result in slugging behavior. For hydrodynamic slugging, the use of a “pseudo-flow” controller in which fluid flow is calculated from an equation for volumetric liquid flow through a valve is the accepted conventional approach. For terrain slugging, the accepted approach is the use of pressure control, wherein the pressure is located upstream of the slug-forming area. For riser slugging, this is at the base of the riser. For both hydrodynamic and terrain slugging, the accepted control schemes for each usually modulate the control valve located upstream of the vessel(s) first receiving produced fluids (e.g., a separator, slugcatcher, free water knock-out, etc.).
Choke valves upstream of a vessel for first receiving produced fluids have been used as manual control valves in fluid processing systems to control slugging. However, such use of the choke valve to control slugging generally results in reduced production. Thus, there is a reluctance to use this to address slugging in practice. Slug control schemes are intended to control slugging behavior by reducing the size (in terms of volume) and/or frequency of slugging behavior, or eliminating it entirely. Maximum production occurs with the valve fully open, but this cannot control or prevent slugging.
Known field-demonstrated control schemes include “pseudo-flow” control, pressure control upstream of the slug forming area, pressure control upstream of the slug forming area cascaded to pseudo-flow control, and composite variable control. Each of these has practical disadvantages. The principle disadvantage of pseudo-flow slug control is that setpoint determination is difficult. Since the pseudo-flow is not an actual physical flow rate, determination of the setpoint is not obvious. Trial and error would be required to determine such a setpoint each time it would need to be adjusted. In the case of slug control via pressure control upstream of the slug-forming area, the principle disadvantage is the cost of the subsea sensor (since the low point is frequently located subsea) and lack of access to the subsea sensor should it fail. Composite variable control uses a calculated variable and therefore as is the case in pseudo-flow slug control, setpoint determination is difficult. It is also as yet unclear whether the underlying calculated variable may need to be periodically redeveloped, making its ongoing use onerous.
It would be desirable to have a mechanism for controlling all types of slugging, including terrain slugging and riser slugging, in a fluid processing system more simply and effectively than has been realized to date.